Excess amounts of a naturally fluorescent molecule found in all living cells could serve as a natural biomarker for cancer, according to Ahmed Heikal, associate professor of bioengineering at Penn State. NADH, or nicotinamide adenine dinucleotide, is a key coenzyme -- a non-protein molecule necessary for the functioning of an enzyme -- found mostly in the inner membrane of a cell's power plant, or mitochondria. It fuels a series of biochemical reactions that involve various enzymes to produce ATP, the major energy source in cells. In the event of disease or a metabolic disorder, these enzymes and their related reactions can become disabled, causing a buildup of unused NADH.